1. Field of the Invention
The present invention relates to Radio Signature Position Determining Entities (RS-PDEs) for mobile phones.
2. Description of the Related Art
Cellular telecommunications networks are well known systems that provide radio service to an ever-growing number of Mobile Stations (MSs) subscribers via a network of Base Stations (BSs), themselves connected to one or more switching nodes. Various types of cellular telecommunications networks exist, including but being not limited to the Time Division Multiple Access (TDMA) based ANSI-41 cellular networks, the Global System for Mobile communications (GSM) networks, the Code Division Multiple Access (CDMA) based networks, and the Third Generation (3G) cellular telecommunications networks (e.g. W-CDMA, CDMA2000, GSM-based EDGE).
In recent years, various countries around the world, including the United States, have introduced legislations governing mobile station location requirements for the wireless subscribers community. According to these requirements, in an emergency situation involving a mobile subscriber, the mobile station would have to be located by the cellular telecommunications network within a given distance, so that law enforcement agencies can use the location information to intervene more efficiently for solving the emergency situation.
Conversely, locating mobile subscribers is also useful for operators of the cellular telecommunications networks, which can use the mobile subscriber location information for providing location-based services to the mobile community, such as for example regional news, location-based information for various proximally located events, or even location-based advertisement for local businesses. There is therefore also an imperative commercial need for locating a mobile station as precisely as possible.
Various methods exist for locating a mobile station in a cellular telecommunications network. The most straightforward method involves including a GPS transceiver into the mobile stations, wherein the GPS transceiver registers the actual position of the mobile station in terms of latitude and longitude. In this implementation, the location information can be provided to the cellular network upon request, or on a regular basis, for both emergency situations and commercial purposes. However, the drawback of the GPS-based method for locating a mobile station is that only a limited portion of the active mobile stations currently comprises a GPS transceiver. Therefore, the GPS-based method does not fulfill the current needs of the cellular industry for locating every active mobile station of a network.
Another way for locating a mobile station is the so-called triangulation method that involves first measuring radio timing parameters of the signals from the mobile station at special listening receivers, typically collocated with three or more base stations, that simultaneously communicate with a central positioning processor. The processor derives the time differences for the received signal as measured at each receiver and then computes the position of the mobile station using a geometric triangulation. Many variants of the triangulation method exist, including some that use the received signal strength instead of the time of arrival parameter for the geometric triangulation. Although certain triangulation methods are relatively precise, the provisioning of large numbers of listening receivers and the requisite communication channels to the central processor translates into significant initial investments for the mobile network operators desiring to implement the triangulation method for locating their cellular subscribers.
Yet another method for locating mobile stations of a cellular telecommunications network is a Radio Signature Positioning System that involves the use of a Radio Signature Positioning Determining Entity (RS-PDE). A typical RS-PDE comprises a processor for computing the position of mobile stations using a positioning algorithm and an associated Radio Signature location database (RS-DB) that contains radio signature data records for all points on a grid that overlays the area of interest. An RS-PDE is used to locate a mobile station by comparing a series of signal strength measurements taken by the mobile station to be located with signal strength measurements stored in the RS-PDE, in order to determine the best match. The best matching record found in the location database of the RS-PDE is assumed to also comprise the current location of the mobile station. FIG. 1 (Prior Art) is an exemplary high-level illustration of such an RS-PDE 100 that comprises a virtual grid 101 that overlaps a physical cellular network 108, the grid 101 having a series of squares 103 which positions are represented by their respective center points. The grid point RS-PDE further comprises a grid point location database 102 where each position of a geographic point like the points 105 of the grid 101 is represented with a measured (and/or predicted, in some implementations) radio signal strength of radio communications that a mobile station receives from its current cell and from its neighboring cells, when the MS is located at a given position represented by that grid point. Typically, the grid point database is populated with radio data measured in the cellular network by a testing mobile station that roams in the area of interest and measures the received signal strength in every point of interest. For example, in the exemplary scenario of FIG. 1, three cells 112, 114, and 116 of the cellular network are represented with their corresponding base stations 118, 120, and 122, although it is understood that a typical cellular network may include a more significant number of cells. The base stations transmit control signals on their respective downlink Digital Control Channels (DCC) 124, 126, and 128. At the grid point location 104, a testing mobile station equipment 130 receives the radio signals on the DCCs 124, 126 and 128 and measures their signal strength levels at that precise location, which geographical coordinates are also recorded via, for example, GPS means. The data collected by the testing mobile station equipment 130 in the point 104, as well as from the other points of the grid, is used to populate the location database records like the exemplary database record 150. Thus, by multiplying the number of measurements performed by the testing equipment 130 at different grid point locations, the location database 102 is populated with multiple records 1–m showing the measured signal strength levels at each one of these multiple locations of the grid 101.
The exemplary location record 150 is shown in FIG. 1 as comprising a Mobile Switching Center Identity (MSC ID) 152 that provides the identity of the MSC servicing the service area where there radio signal strength measurements were registered, and a serving cell identity 154 providing the identity of the serving cell 112 used for the radio signal strength measurements. The location record 150 further comprises the actual position 156, in terms of latitude and longitude, that may have been measured using GPS means included in the testing mobile station equipment 130, and a series of n radio signal strength measurements 158 performed on the DCCs of the serving and neighboring cells of the cellular telecommunications network. For example, the parameter DCC—CH1—SS 1581 may represent the received signal strength measured on the digital control channel of the serving cell 112, while the parameter DCC—CH2—SS 1582 represents the signal strength measured on the digital control channel of a first cell 114 of n cells adjacent or geographically close to the serving cell 112. In order to have reliable measurements, the number n is typically selected between 6 and 12, thus allowing radio signal strength measurements to be done first on the serving cell, and further to 5–11 adjacent or closely located cells.
Once the location database 102 is populated with signal strength data for each grid point of the grid 101, a Mobile Positioning Center (MPC) 162 with access to the RS-PDE's processor 160 and to the location database 102 may find the location of any mobile station of the cellular network for responding to emergency situations as described hereinbefore, or simply for a commercial sake. The MPC 162 sends a series of D—CCs signal strength measurements taken by the mobile station to be located to the RS-PDE, where an RS-PDE Processor 162 compares these measurements with the records of the location database 102, for determining the best match. The best matching record found in the location database 102 is assumed to represent the current location of the mobile station, which is provided in terms of position information including the longitude and latitude parameters from the positioning field 206 of the best matching record 150.
The RSPDE is a reliable means for providing mobile station location information at reasonable costs for the network operator. However, the level of the radio signal strength data that is initially measured during the field test of the testing mobile station equipment 130 and that populates the database 102 forms the basis for the location algorithm. It was detected that over long periods of times the measured signal strength in a given location may vary due to i) the continue adjustments of the cellular network's topology (e.g. the cell definitions may be changed by the network operator to adapt to the changing traffic load), and to ii) seasonal and environmental changes (e.g. during the fall, trees of a forest lose their leaves thus changing the level of the measured signal strength in the area, an imposing building is constructed at the given location, etc). Since radio propagation conditions change with respect to the initial time where the testing equipment 130 collected the initial radio data, the mentioned factors introduce an error in the calculation of the position based on the signal strength, which negatively influence the performance of the grid point RS-PDE.
Although there is no prior art solution as the one proposed hereinafter for solving the above-mentioned deficiencies, the co-owned patent application entitled “Determination of Mobile Station Location”, Ser. No. 10/095,958, filed in U.S. Patent and Trademark Office on Mar. 13, 2002 in the name of Gennady Bayder, hereinafter called Bayder bears some relation with the field present invention. Bayder teaches a system, method and processing unit for mobile station location determination. In Bayder, Mobile Assisted Handoff (MAHO) measurements are sent to a processing unit that also retrieves the corresponding transmitted signal strengths and electromagnetic field distribution functions for the relevant base stations. The location of the mobile station is then determined by minimising the following formula:       F    ⁡          (              γ        ,        x        ,        y            )        =            ∑              j        =        1            m        ⁢          M      ⁡              (                              P                          R              ⁢                                                          ⁢              j                                -                      γ            ·                          P                              T                ⁢                                                                  ⁢                j                                      ·                                          G                j                            ⁡                              (                                  x                  ,                  y                                )                                                    )            where m is the number of relevant base stations, M is an optimisation metric (such as (εj)2 or |εj|) PRj is received signal strength, PTj transmitted signal strength, γ attenuation (e.g. in the mobile station), and Gj(x,y) the electromagnetic field distribution function. Bayder's teaching is limited to a method and corresponding system for calculating a location of a mobile station using a specific algorithm defined by the above-mentioned formula, and therefore Bayder fails to teach or suggest any method or system for ensuring the consistency and the reliability of a grid point RS-PDE.
The co-owned patent application entitled “System and Method of Estimating the Position of a Mobile Terminal in a Radio Telecommunications Network”, filed in U.S. Patent and Trademark Office on Jan. 10, 2002, in the names of Havish Koorapaty, Wang Wade, Zhu Guwei, and Panakanti Viswanath, hereinafter called Koorapaty, also bears some relation with the field of the present invention. Koorapaty teaches a system and method of estimating the position of a mobile terminal operating in a radio telecommunications network. Expected received signal strength values are predicted by a computer prediction tool, and/or are measured by a test mobile terminal from base station transceivers. The predicted and measured received signal strength values are then tagged to indicate whether each value was predicted or measured. The received signal strength values are then stored at a plurality of locations in a database. When the received signal strength measurements are received from the mobile terminal being located, a covariance matrix is used to compute metrics for the locations in the database. If more than a threshold percentage of the locations were populated with measured values, a maximum likelihood estimator is used to estimate the position of the mobile terminal. Otherwise, if fewer then a threshold percentage of locations were populated with measured values, a minimum mean square error estimator is used to estimate the position. Koorapaty is limited to teaching a system and method for estimating the position of the mobile terminal, and therefore fails to teach or suggest anything with relation to the reliability of the location data in a grid point RS-PDE.
Accordingly, it should be readily appreciated that in order to overcome the deficiencies and shortcomings of the existing RS-PDE implementations, it would be advantageous to have a simple yet efficient method and system for insuring the continuous reliability of the signal strength data that is used by a grid point RS-PDE system. The present invention provides such a method and system.